High speed non-impact printing

ABSTRACT

Printing is effected by applying a shaped electrical field as a pulse of short duration between a donor sheet and a closely adjacent recipient sheet. The surface of the donor sheet closest to the recipient sheet carries electrically conductive particles of a printing material or pigment dispersed in or on a high resistance medium. The electrical field causes a current flow which transfers a sufficient charge to the printing material to cause it to move to the recipient sheet under the influence of the field.

United States Patent Haeberle et al. Oct. 14, 1975 [54] HIGH SPEED NON-IMPACT PRINTING 2,901,374 8/1959 Grunlach 117 175 2,993,816 7 1961 [75] Inventors: Robert W. Haeberle; Donald J. J. 3,104 985 9x963 Lennon, both of Acton; Harvey G. 35093088 4/197O Schl ifst n, t t all of Mass. 3,707,391 12 1972 Goffe 117 226 Appl. No.: 280,460

Related US. Application Data Continuation of Ser. No. 32,484, April 13, 1970, abandoned, which is a division of Ser. No. 693,965, Dec. 27, 1967, Pat. No. 3,550,153.

Primary Examiner-Michael F. Esposito Attorney, Agent, or FirmKenway & Jenney ABSTRACT Printing is effected by applying a shaped electrical field as a pulse of short duration between a donor sheet and a closely adjacent recipient sheet. The surface of the donor sheet closest to the recipient sheet [52] U.S.Cl. 346/139 A carrles electrlcally conductlve partlcles of a prlntmg l' material or pigment dispersed in or on a high resis 0 are tance medium. The electrical field causes a current flow which transfers a sufficient charge to the printing material to cause it to move to the recipient sheet [56] References under the influence of the field.

UNITED STATES PATENTS 2,788,296 4/1957 Louis 117/226 3 Claims, 6 Drawing Figures US. Patent Oct. 14, 1975 FIG. I

FIG. 2

FIG. 5

HIGH SPEED NON-IMPACT PRINTING This is a continuation of application Ser. No. 32,484, filed Apr. 13, 1970 now abandoned which is a division of application Ser. No. 693,965 filed Dec. 27, 1967 now U.S. Pat. No. 3,550,153.

BACKGROUND OF THE INVENTION This invention relates generally to printing methods that employ an electrical field to move and cause adherence of mobile, particulate printing material to a recipient sheet. In such methods the outline of the material so attracted bears the desired image configuration.

In some methods, exemplified by the patent to Oster U.S. Pat. No. 3,145,156, and the patent to Carlson U.S. Pat. No. 3,257,222, printing particles are initially dispersed throughout a donor sheet from which the portion defining the image configuration is selectively transferred by an electrostatic field. These methods involve two steps, of which the first is the precharging of the particles on the donor sheet and the second is the imposition of an electrostatic field to develop the necessary force to transfer the particles so charged to the recipient sheet. The Carlson method illustrates a variant in which the image is defined by the shape of the electrode employed in the first step. A dry toner is employed on the donor sheet. The Oster patent illustrates a variant in which the image is defined by the shape of the electrode employed in the second step, or a stencil placed over such electrode. The method uses charged mobile particles in a fluid phase. The Carlson and Oster methods begin with printing material distributed throughout the donor sheet in a manner analogous to the carbon on a sheet of carbon paper. They are characterized by the use of a character-shaped electrode. 7

Other methods are characterized by a sheet that is selectively charged to define the image configuration, then developed by placing the sheet close to a source of precharged printing particles called toner, such as a cascade of loose particles, whereby the particles adhere only to the charged areas. These latter methods employ either two or three distinct steps. Ordinarily, the sheet so charged is an intermediate sheet forming a part of the printing apparatus. In this case, there are three steps: first, the selective charging of the intermediate sheet, second, the cascade development (or its equivalent such as loop development, powder cloud development, brush development or magnetic development), and third, the transfer of the particles retained on the intermediate sheet to a recipient sheet. In a simplified arrangement, the final step is eliminated and a recipient sheet of paper replaces the intermediate sheet in the earlier steps.

The patent to Byrne U.S. Pat. No. 2,951,443 illustrates a three-step method in which the first step utilizes the photoconductivity of an intermediate sheet to produce a latent, charged image. Exposure is by optical projection. The patent to Rheinfrank U.S. Pat. No. 3,093,039 is similar but employs contact printing. The patent to Green U.S. Pat. No. 2,953,470 illustrates the variant in which the first step involves making the intermediate sheet with an image-defining stencil, and producing a corona discharge that leaves a latent, charged image only on the areas exposed through the stencil.

All of the foregoing methods involve successive steps or stages with attendant complications of apparatus and inherent limitations on the speed of operation. An object of this invention is to provide a printing method that is at once simpler and substantially faster. Applications, for example, are on-line computer print-out and facsimile transmission. In particular, it is desired to provide a method suitable for code, alphanumeric, graphic or type composition.

A further object is to secure the foregoing advantages without sacrifice of the recognized advantages of electrostatic printing techniques, notably high image fidelity, uniform high density of printing and adaptability to the use of inexpensive paper recipient sheets, or the like.

SUMMARY OF THE INVENTION This invention achieves the foregoing objects by a simple single-step process involving both the rapid, selective charging of image-defining printing or pigment particles on a donor sheet and the physical transfer of such particles to a recipient sheet. It is a characteristic of this method that the same pulsed field employed to produce a charge on the particles is sufficient to cause their immediate transfer to the recipient sheet.

An additional feature is the use of an efficient charging technique whereby a charge can be imparted to the particles in an extremely brief space of time, thus permitting imposition of a strong accelerating force upon them and permitting their transfer from the donor sheet to the recipient sheet within this brief time interval. This technique features the use of a donor sheet in or upon which electrically-conducting printing particles are dispersed or embedded in or upon a medium or matrix of material having high electrical resistance. There is sufficient conductivity in the matrix so that the presence of the applied electric field, sufficient current is conducted from or to the pigment particles to charge them to a value which, coacting with the same field, detaches them and transfers them to the recipient sheet.

At the same time, provision is made to prevent a breakdown or discharge between the donor sheet and the recipient sheet, since such breakdown produces a transfer of charge without an attendant transfer of pigmerit.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation in cross section of a simplified arrangement of electrodes and donor and recipient sheets for printing in accordance with this invention.

FIG. 2 is a representation of a variant in which no air gap or discharge suppressing sheet is employed.

FIG. 3 is a representation of a variant in which images of closely adjacent characters are printed.

FIG. 4 is a representation of a variant in which images of a character are simultaneously printed on two recipient sheets.

FIG. 5 is a diagrammatic cross section of one form of source sheet.

FIG. 6 is a diagrammatic cross section of one form of image defining electrode.

It is to be understood that the drawings are entirely schematic and in many ways exaggerated. The arrangements illustrated are not to be taken as definitive of the system but to serve mainly as a graphical memorandum of the essential aspects of the system. Unless otherwise stated in the following specification the donor sheet and recipient sheet may be in contact or closely spaced, as may be the relationship of the electrodes to these sheets. It is specifically contemplated that an air (or vacuum) gap may separate either electrode from the sheets, or the sheets from each other. The representation of the pulse as a square wave does not imply that the pulse must be square, as it will be apparent to those skilled in the art that the mechanisms described do not depend on the pulse being of any precise particular shape.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, one example of means for carrying out the method of this invention is shown in simplified form. A pair of metallic electrodes 12 and 14 are situated on opposite sides of a composite sheet assembly 16. The assembly 16 comprises three sheets, a discharge suppressing insulator sheet 18 typically of Mylar polyester film, a polyethylene terephthalate resin, or its equivalent, a donor sheet 20 comprising in this case a fabric substrate coated with or bearing mobile particles of a high resistance binding or matrix material in which are embedded electrically-conductive printing or pigment particles, and a recipient sheet 22 which is typically ordinary paper, e.g., an all rag carbonizing tissue or news stock. A paper having a smooth hard coating is preferably not used as it does not hold onto the particles well. I

The electrode 14 has a flat smooth surface in contact with the sheet 18. The electrode or printing head 12 in this embodiment has a character 24 formed in relief facing the sheet 22. An air gap 26 may separate the character 24 and the sheet 22, and is preferably only as large as necessary to prevent electrical breakdown when a printing pulse 28 is applied between terminals 30 and 32.

An air gap in this location is a practical necessity in apparatus in which the character defining electrode 12 is carried on a rapidly rotating print drum, as would be required in a high speed character printing device. As stated earlier, an air gap may also separate electrode 14 from the backing sheet 18, as well as the two sheets.

FIG. 6 shows another suitable form of printing head 12a, viewed on the side facing the recipient sheet. The letter C is metallic and comprises an image in relief integral with the body of the electrode. A layer of insulating material 33 fills the space around the character and is flush with its surface. This layer may be formed by uniformly coating the electrode with an insulating plastic material, solidifying the plastic and grinding the coated surface to a sufficient depth to expose the sur face of the character.

The foregoing alternative forms of printing heads may be employed in any of the embodiments hereinafter described.

The donor sheet 20 in a preferred and typical embodiment of the invention, given here by way of example only, is of relatively high resistance and is prepared as follows.

FORMULA l by Weight Source Carbon 6.0 Cabot Corp. Mogul A" fluffy type Polystyrene 3.0 Dow Chemical Corp. PS-3 losol Black Dye 0.8 National Aniline -Continued FORMULA 1 7r by Weight Source losol Yellow Dye 0.8 National Aniline Iosol Red Dye 0.8 National Aniline Toluene 88.6

The carbon is dispersed in a solution of the resin and dyes using a well known sand milling technique. The resulting suspension is dried and the residue ground. The ground particles or granules are impregnated in a woven-nylon substrate fabric sheet in a sand mill. The impregnated sheet may have the form of a ribbon strip or any other suitable sheet form.

The resistivity of the sheet 20 is conveniently measured by removing the sheets 18 and 22, closing upon the sheet 20 a pair of test electrodes having directly opposed, identical A inch flat round surfaces, and measuring the dc resistance between the electrodes. The resistivity can be varied by changing the ratio of the conductive pgiment to the insulating resin. The polystyrene is well known as an insulating material, but in the instant application it is rendered slightly conductive by the presence of the dyes dissolved or dispersed in the resin. It may be noted that the toners generally used in the prior art methods are not sufficiently conductive for practical use in the present method.

Using the example of Formula 1 wherein the sheet 20 had a resistivity of 2 X l0 ohm-cm. in the apparatus of FIG. 1 with an air gap 26 of 2.0 mils and a Mylar sheet 18 if 35 X 10 inch thickness, the image of the character 24 was printed upon the recipient sheet 22 by applying a pulse 28 of 1300 volts for nanoseconds, i.e., 150 X 10 second. This extremely brief time interval is indicative of the great printing speed of which this method is capable.

The electrical response to the applied voltage pulse of the material impregnating the sheet 20 broadly comprises two phenomena: first, a flow of current, in the presence of the electrical field, through the matrix of high resistivity binding material, resulting in an electrical charge upon the pigment or carbon-containing granules at or near the surface of the sheet 20, and second, the detachment of a significant number of charged granules from the sheet 20 and their transfer to the sheet 22.

Once the granules are on the sheet 22 they may, if desired or necessary, be fixed thereto, for instance by heat, if the particles are fusible, or by means of solvents, lacquer or otherwise in accordance with conventional technology. a

The method described above in relation to FIG. 1 may be varied in several added respects. For example, the substrate fabric of the donor sheet may be another cloth material, or a paper of plastic film. It might comprise a part or all of the high resistivity medium in which the printing particles are dispersed and through which the current flows to charge the characters. Also, various conventional ribbon inking or coating processes may be employed. The high resistivity matrix containing the conductive particles may be applied as a surface coating rather than as an impregnant to the substrate, as shown in FIG. 5. This figure shows a plastic film substrate 34 coated with a matrix 36 of high electrical resistivity, in which there are dispersed mobile, electrically conductive pigment particles 38.

The method described above, as well as the variants hereinafter described, may be employed under vacuum conditions, e.g., down to 4 X l- Torr, as well as in the presence of atmospheric air or other gases. Thus, although the term air gap is employed herein, this does not necessarily imply the presence of air and is intended only to define a spatial separation of the elements. 7

The air gap 26 and the insulating sheet 18 together serve to prevent arcing which discharges a part of the charge upon the pigment-containing particles and prevents the development of sufficient force in the electrical field to cause their transfer. Under certain conditions the air gap or the sheet 18, or both may be eliminated. In FIG. 2, both have been eliminated in an arrangementin which a pulse 40 of lower amplitude than the pulse 28 is sufficient to cause printing to occur. In this case a donor sheet 42 of relatively low resistivity is used. It is prepared as follows.

FORMULA 2 The carbon is dispersed in the alcohol and impregnated in a woven nylon ribbon substrate fabric in a sand mill. The ribbon is driedand its surface mechanically abraded in a sand mill. In this instance, the high resistivity path or matrix by means of which the current flows to charge the carbon comprises the substrate fibers rather than a plastic coating.

Using the example of Formula 2 wherein the sheet 42 had a resistivity of 5 X ohm-cm. in the apparatus of FIG. 2, the image of the character 24 was printed upon the recipient sheet 22 by applying a pulse 40 of 350 volts for 30 nanoseconds, i.e., 30 X 10 second. Satisfactory results were also obtained with 250 volt, 4O nanosecond pulses, without an air gap.

With a 5 X 10 ohm cm. ribbon and no insulating sheet 18, higher pulse amplitudes can be employed if there is an air gap similar to that shown in FIG. 1. For example, an air gap of 0.001 inch can be employed in conjunction with 500 volt, 40 nanosecond pulses.

The factor of image definition merits consideration in selecting the proper voltage level and pulse duration for image transfer. This is illustrated by FIG. 3 which shows elementary apparatus for printing a character 44 and an adjacent character 46 on the recipient sheet 22, these characters being in relief on an electrode 48. Plane surfaced electrodes 50 and 52 are mounted beneath the respective characters and are separated from each other by a 0.002 inch air gap 53. An insulator sheet 54 is also employed as described in the above examples.

Selective printing from characters 44 or 46 is provided by applying the electrical pulses selectively to one or the other of the lower electrodes 50 or 52. With an air gap 58 of 0.002 inch separating the characters 44 and 46 from the recipient sheet 22, and with a donor sheet having a resistivity of 1.8 X 10 ohm-cm, a

pulse of 1,000 volts, nanosecond duration, will result in selective printing. With a donor sheet 20 of 2.0 X 10 ohm-cm resistivity a pulseof 1300 volts, 150 nanosecond duration, will result in selective printing.

Adequate image selectively at lower voltage was also obtained using a donor sheet 20 having a resistivity of 4 X 10 ohm-cm; at these levels the air gap 58 and the sheet 54 were eliminated, and the pulse amplitude was reduced to about 750 volts.

FIG. 4 illustrates a variation of the invention in which printing is simultaneously effected on two recipient sheets 22a and 22b. In the form shown, the donor sheet 42 of relatively low resistivity was employed, and 400 volt, I10 nanosecond pulses 60 were applied between the electrodes 12 and 14. Upon application of the pulse the printing granules in or near the upper surface of the donor sheet became charged with one polarity and moved upwardly to the sheet 22a, while at the same time the granules in or near the lower surface of the donor sheet became charged with the opposite polarity and moved downwardly to the sheet 22b.

As suggested by FIG. 4, in the embodiments of FIGS. 1 to 3 it is possible to have the characters 24, 44 or 46 on either electrode: that is, the image-defining electrode or printing head may be adjacent either the donor sheet or the recipient sheet, the object in any case being to provide a field shaped or bounded to define the image to be printed. The side chosen is generally that which affords the best definition of the character on the recipient sheet. In some cases the same character may be situated on the surface of both electrodes where considerations of definition so indicate. Also, the opposing electrode surfaces may be flat, in which case the shape of the character printed corresponds to the electrode surface outline.

In the practice of this invention it is necessary that the electrical field shall retain the character configuration to be printed throughout the region from the character defining electrode to and including the space between the opposing surfaces of the donor and recipient sheets. This may restrict the choice of materials for the substrate of the donor sheet in some variants of the process, since any conductive material causes or tends to cause equilization of the electrical potential of points within it and destroys the field configuration. This would be of no concern in the variants of FIGS. 1 to 3 because the substrate of the donor sheet is not situated in the above region. However, in the variant of FIG. 4 the substrate of the donor sheet 42 lies within the region between the character 24 and the surface of the recipient sheet 22b, and printing can be effected on the latter only if that substrate is of sufficiently resistive material.

It will be understood that this invention is useful for printing code symbols, such as binary codes for example, as well as alphanumeric characters or any other predetermined configurations, and also the printing of half tones, blocks or lines.

The conductive pigments used in the practice of this invention may take various forms, including powdered metals, e.g., brass, and metal oxides, e.g., lead dioxide.

Other types of dyes, high dielectric resins and solvents may also be used.

It has also been demonstrated that printing by the system herein described may be accomplished with a multi-layer assembly of several donor sheets and several interspaced recipient sheets, provided that all the sheets are sufficiently resistive so as not to distort the shape of the electrical-field.

Other variations in the methods employed, as well as in the formulations or compositions of the, components used, will also occur to one skilled in the art of electrostatic or electrophoretic printing and familiar with the principles herein disclosed. These are intended to fall within the scope of the appended claims to the extent permitted by the context.

We claim:

l. A donor sheet for electrical pulse printing having a conductive high resistance self-supporting portion and a plurality of finely divided, electrically conductive uncharged mobile printing particles attached lightly to and dispersed over a first surface of said portion, said sheet having an electrical resistivity of at least X 10 ohm-cm and having sufficient conductivity to conduct a current between said particles and the opposite surface of said self-supporting portion during the presence of an image shaped electrical field of short duration extending through said surfaces so that those particles located within said image shaped field accumulate charge of a sufficient magnitude and sign to cause them to detach from said donor sheet during the presence of said electrical field.

2. A donor sheet according to claim 1, in which the printing particles consist essentially of carbon 3. In a high speed non-impact printing means wherein image forming pigment is transferred to a recipient sheet in a single step by the application of an image shaped pulsed electric field of short duration to a source of conductive printing particles to transfer printing vparticles from said source to said recipient sheet, an improved source comprising:

a donor sheet having a conductive high resistance portion and a plurality of finely divided, electrically conductive uncharged mobile printing particles attached to and dispersed over a first surface of said portion with light adhesion. permitting mobility away from said surface when said particles are charged in the presence of said field, said'sheet having an electrical resistivity of at least 5 X 10 ohm-cm and having sufficient conductivity to conduct current between said particles and the opposite surface of said high resistance portion during the presence of said field extending through said surfaces so that those particles located within said field accumulate charge of a sufficient magnitude and sign to cause then to transfer from said donor sheet to said recipient sheet. 

1. A donor sheet for electrical pulse printing having a conductive high resistance self-supporting portion and a plurality of finely divided, electrically conductive uncharged mobile printing particles attached lightly to and dispersed over a first surface of said portion, said sheet having an electrical resistivity of at least 5 X 106 ohm-cm and having sufficient conductivity to conduct a current between said particles and the opposite surface of said self-supporting portion during the presence of an image shaped electrical field of short duration extending through said surfaces so that those particles located within said image shaped field accumulate charge of a sufficient magnitude and sign to cause them to detach from said donor sheet during the presence of said electrical field.
 2. A donor sheet according to claim 1, in which the printing particles consist essentially of carbon.
 3. In a high speed non-impact printing means wherein image forming pigment is transferred to a recipient sheet in a single step by the application of an image shaped pulsed electric field of short duration to a source of conductive printing particles to transfer printing particles from said source to said recipient sheet, an improved source comprising: a donor sheet having a conductive high resistance portion and a plurality of finely divided, electrically conductive uncharged mobile printing particles attached to and dispersed over a first surface of said portion with light adhesion permitting mobility away from said surface when said particles are charged in the presence of said field, said sheet having an electrical resistivity of at least 5 X 106 ohm-cm and having sufficient conductivity to conduct current between said particles and the opposite surface of said high resistance portion during the presence of said field extending through said surfaces so that those particles located within said field accumulate charge of a sufficient magnitude and sign to cause then to transfer from said donor sheet to said recipient sheet. 